In a semiconductor device (power module) which uses a power semiconductor chip such as a silicon-carbide metal oxide semiconductor-field effect transistor (SiC MOSFET) or a silicon insulated gate bipolar transistor (Si IGBT), there is a need to efficiently dissipate heat generated from a semiconductor chip, and keep a temperature of a semiconductor chip at a predetermined temperature or lower.
Conventionally, provided is a power module in which a semiconductor chip is bonded to one of surfaces of a so-called insulating substrate which is formed by integration of: an insulating ceramic plate of high thermal conductivity such as silicon nitride, silicon aluminum, or alumina; and conductive plates formed of a metal of high thermal conductivity such as aluminum or copper (including an alloy thereof, and the same shall apply hereinafter), which are provided on opposite surfaces of the insulating ceramic plate, via a bonding member such as solder, and a cooler is bonded directly or indirectly to the other of the surfaces of the insulating substrate via a bonding member such as solder.
However, depending on a use condition, a thermal stress is caused due to a difference in thermal expansion coefficient between the insulating substrate and the cooler, and a crack occurs in a bonding member which bonds the insulating substrate and the cooler, so that satisfactory heat dissipation properties cannot be maintained throughout a required lifetime in some cases.
Thus, in order to solve the foregoing problems, there is made a suggestion that a stress relieving member is placed between the insulating substrate and the cooler (refer to Patent Document 1, for example).
A stress relieving member in Patent Document 1 includes an aluminum plate member which has a thickness of not less than 0.3 mm and not more than 3 mm and has a plurality of through holes which are formed therein to serve as stress accommodating spaces. The stress relieving member is brazed to the insulating substrate and a heat sink. By virtue of actions of the stress accommodating spaces, the stress relieving member is deformed, so that a thermal stress is relieved.